The present invention is directed generally to the field of expandable intraluminal prostheses, commonly referred to as stents, and more particularly to a composite device including a thermally expandable stent and an open mesh stent cover.
Stents are typically used as adjuncts to percutaneous transluminal balloon angioplasty procedures, in the treatment of occluded or partially occluded arteries and other blood vessels. In a typical balloon angioplasty procedure, a guiding catheter or sheath is percutaneously introduced into the cardiovascular system of a patient through the femoral arteries and advanced through the vasculature until the distal end of the guiding catheter is positioned at a point proximal to the lesion site. A guidewire and a dilatation catheter having a balloon on the distal end are introduced through the guiding catheter with the guidewire sliding within the dilatation catheter. The guidewire is first advanced out of the guiding catheter into the patient""s vasculature and is directed across the arterial lesion. The dilatation catheter is subsequently advanced over the previously advanced guidewire until the dilatation balloon is properly positioned across the arterial lesion. Once in position across the lesion, the expandable balloon is inflated to a predetermined size with a radiopaque liquid at relatively high pressure to radially compress the atherosclerotic plaque of the lesion against the inside of the artery wall and thereby dilate the lumen of the artery. The balloon is then deflated to a small profile so that the dilatation catheter can be withdrawn from the patient""s vasculature and blood flow resumed through the dilated artery. As should be appreciated by those skilled in the art, while the above-described procedure is typical, it is not the only method used in angioplasty.
Balloon angioplasty sometimes results in short or long term failure. That is, vessels may abruptly close shortly after the procedure or gradual restenosis may occur for months thereafter. To counter the tendency of recurrent vessel occlusion following angioplasty, implantable intravascular prostheses, commonly referred to as stents, have emerged as a means by which to achieve long term vessel patency. Stated simply, a stent functions as permanent scaffolding to structurally support the vessel wall and thereby maintain luminal patency. Stents are typically small expandable metallic tubes having interconnecting spans and struts which form a generally open cellular construction. Stents are transported to a lesion site by means of a delivery catheter.
There are several types of stents mainly, balloon expandable stents, spring-like self-expandable stents, or thermally expandable stents. Balloon expandable stents are delivered by means of a dilitation catheter and are plastically deformed by means of an expandable member, such as an inflation balloon, from a small initial diameter to a larger expanded diameter. Self-expanding stents are formed as spring elements which are radially compressible about a delivery catheter. A compressed self-expanding stent is typically held in the compressed state by a delivery sheath. Upon delivery to a lesion site, the delivery sheath is retracted allowing the stent to expand. Thermally expandable stents are formed from shape memory alloys which posses the ability to expand from a small initial diameter to a second larger diameter upon the application of heat to the alloy. Although each method of stent expansion is effective, self-expanding stents tend to be difficult to deploy accurately, and balloon expandable stents may, in rare circumstances, inflict undesirable trauma on particularly fragile vessels.
Details of prior art expandable stents can be found in U.S. Pat. No. 3,868,956 (Alfidi et al.); U.S. Pat. No. 4,512,1338 (Balko et al.); U.S. Pat. No. 4,553,545 (Maass, et al.); U.S. Pat. No. 4,733,665 (Palmaz); U.S. Pat. No. 4,762,128 (Rosenbluth); U.S. Pat. No. 4,800,882 (Gianturco); U.S. Pat. No. 5,514,154 (Lau, et al.); U.S. Pat. No. 5,421,955 (Lau et al.); U.S. Pat. No. 5,603,721 (Lau et al.); U.S. Pat. No. 4,655,772 (Wallsten); U.S. Pat. No. 4,739,762 (Palmaz); and U.S. Pat. No. 5,569,295 (Lam), which are hereby incorporated by reference.
While stents alone perform adequately for the purpose of holding open otherwise occluded, partially occluded, or weakened blood vessels, due to the open structure of a stent there is a tendency for a stent to permit the passage of material through the stent body. Such material may include excessive cell or tissue growth, thrombus formations, and plaque. These materials may have a tendency to block or otherwise re-occlude the open vessel.
One technique to reduce the susceptibility for materials to pass through the wall of a deployed stent includes providing the stent with an outer covering formed from a bio-compatible polymer surrounding the open stent construction. One such material commonly used for this purpose is GORE-TEX Vascular Graft (W. L. Gore and Associates, Inc, Flagstaff, Ariz.), which is a micro porous polymer film. The stent cover, however, presents somewhat of a problem in that some transfer of cellular material through the cover is generally desirable. If the cover material is porous, then cells, tissue, and capillaries can penetrate the pores, allowing the blood vessel to be re-endothelialized with new healthy tissue. However, if the covering is too porous, there may be a tendency for diseased tissue to transfer itself to the newly created intima and damage healthy tissue. Micro porous materials like GORE-TEX Vascular Graft have pore sizes on the order of 10-100 microns and are effective at preventing diseased tissue ingrowth. However, due to the very small pore size, re-endothelialization with new healthy tissue may be somewhat compromised. Also, the stent cover material must be sufficiently flexible and expandable to permit deployment of the stent from its initial diameter to its deployed diameter. Many non-porous and micro-porous films, when formed as tubular covers, do not readily expand in the radial direction. For this reason most prior art stent covers are either foldable or have an overlapping slidable design to permit stent expansion. Typically, the covers are attached to the stent along a single seam running along the length of the stent. Thus, the covers may slip or deploy non-uniformly during stent expansion.
What is needed, therefore, is a composite intraluminal prosthesis including a stent and a macro-scale outer mesh covering which fits over the stent and presents little or no resistance to radial expansion. In addition, the cell size of the mesh cover should be large enough to allow re-endothelialization of the diseased blood vessel wall with healthy tissue and should be small enough to prevent plaque prolapse or the growth of diseased tissue within the open structure of the stent. Further, it maybe desirable for the underlying stent to be of the thermally expandable type in order to minimize vessel trauma which may be caused by balloon expandable stents or other self-expanding stents. The present invention satisfies these and other needs.
The present invention is an intraluminal prosthesis or covered stent comprising a tubular expandable stent having an open cell mesh stent cover. The stent has an exterior surface, a luminal surface, and plurality of openings through the wall to provide scaffolding support for the stent cover. The stent can be formed from a two-way shape memory alloy. Particular use is made of the thermal expansion properties of the shape memory alloy to provide a stent which expands and implants itself within a blood vessel without inflicting trauma to the vessel lumen. The stent cover can be formed from a mesh having an open, square or diamond cell pattern. The particular cell size of the mesh is selected so as to prevent plaque prolapse and the ingrowth of diseased tissue through the openings in the stent, thereby inhibiting possible re-occlusion of the vessel. The mesh cell size is further selected to allow for the re-endothelialization of the vessel wall with healthy tissue. The stent cover is able to expand and contract with the underlying stent without becoming loose if contracted and without exerting significant resistance to radial expansion.
Other features and advantages of the present invention will become apparent from the following detailed description.